A terahertz (THz) wave may be undeveloped resources having a bandwidth from 100 gigahertz (GHz) to 10 THz and correspond to an intermediate area between an infrared light and a millimeter wave. Due to a high technical entry barrier difficult to overcome with a current technology level, the THz has been an unexplored technical field recognized as a THz gap. THz wave technology may belong to an optical field at the early stage. However, with developments of nano-level electronic element/substance technologies, the THz wave technology has been applied as a combination of optical engineering technology and electronic engineering technology.
In terms of the electronic engineering technology, research on a passive element such as a resonant tunneling diode (RTD) and a schottky barrier diode (SBD) has been actively conducted. Recently, III-V heterojunction bipolar transistor (HBT), and high-electron-mobility transistor (HEMT) elements has succeeded in an access to a block frequency of 1 THz, which allows a sub-THz operation. In terms of the optical engineering technology, developments of elements such as a photoconductive switch, optical rectification, difference-frequency generation (DFG), optical parametric, terahertz-quantum cascade lasers (THz-QCL), and uni-traveling-carrier photodiodes (UTC-PD) have been actualized to advance the THz technology.
Current nano-transistor technology has been continuously downscaled to 20 nanometers (nm) for achieving an increased operational frequency. However, an operation at a band of 500 GHz or more may be restricted despite a channel reduction in a transit-mode. A new concept element provided to solve this issue, a plasma wave transistor (PWT) may operate at a velocity corresponding to 10 to 100 times of a transit-mode electron drift velocity using a plasma wave defined as a space-time oscillating wave of a channel electron density. Research on a PWT for THz emission and detection element operating in a frequency domain higher than a block frequency of a transistor based on a plasma resonance phenomenon of a two-dimensional (2D) channel electron density has been conducted all over the world to be used as technology for filling the THz gap.
Since 1993 when Michael Shur, a professor at Rensselaer Polytechnic Institute (RPI) in the United States proposed for the first time, research on implementation and application of the THz emission and detection element using the PWT has been conducted in universities in the U.S., Japan, and Europe for about 10 years. However, there still exists a technical difficulty in evaluating the THz element and a property of the THz element at a commercialization level.
Technology for reaching a THz range using a resonator for a plasma wave, a correlation between a frequency of the plasma wave and a drain-to-source current, and a relationship between a plasma wave velocity and a drift velocity are theoretically explained in the following prior art documents.
(Prior art 1) Terahertz Emitters, Detectors and Sensors: Current Status and Future Prospects, M. Ghanashyam Krishna, Sachin D. Kshirsagar and Surya P. Tewari, intech.
(Prior art 2) Terahertz Resonant Detection by Plasma Waves in Nanometric Transistors, F. Teppe, A. El Fatimy, S. Boubanga, D. Seliuta, G. Valusis, B. Chenaud and W. Knap, Proceedings of the 13th International Symposium on ultrafast phenomena in semi-conductors (UFPS).